Various types of hydrostatic continuously variable transmissions have been known and implemented. For example, patent document 1, patent document 2 and patent document 3 proposed by the present applicant disclose a hydrostatic continuously variable transmission. This hydrostatic continuously variable transmission disclosed by these patent documents comprises a cam plate type axial plunger pump, a cam plate type axial plunger motor, and a hydraulic closed circuit for connecting the outlet port and inlet port of the cam plate type axial plunger pump to the inlet port and outlet port of the cam plate type axial plunger motor, a pump cam plate member is driven by an engine, a pump cylinder and a motor cylinder are joined together and connected to an output shaft, and the rotation of a motor cam plate member is restricted, and the angle of the motor cam plate can be changed.
In the hydrostatic continuously variable transmission thus constituted, it has been known that a clutch valve for connecting and disconnecting a high pressure side oil passage and a low pressure side oil passage constituting the hydraulic closed circuit is installed to control the size of rotation drive force to be transmitted from the hydraulic pump to the hydraulic motor and to cut off the transmission of this rotation. For example, patent document 3 discloses an automatic clutch device which comprises this clutch valve. This clutch valve has a spring (urging means) for urging it in the opening direction and a hydraulic governor for generating governor oil pressure corresponding to the input engine speed and is opened or closed according to the urging force of the spring and the force of the governor (governor oil pressure) to connect or disconnect the high pressure side oil passage and the low pressure side oil passage.
[patent document 1] JP-A No. 42446/1994
[patent document 2] JP-A No. 2920772
[patent document 3] JP-A No. 100909/1997
In the above clutch valve, when the input engine speed is low (for example, when the engine idles), the urging force of the spring prevails and the clutch valve is opened (the clutch is disconnected) and when the input engine speed is high, the force of the governor prevails and the clutch valve is closed (the clutch is connected). However, at around an input engine speed at which the clutch valve moves in the opening direction, forces in the direction of disconnection and the direction of connection compete with each other. When force in the direction of disconnection grows all of the sudden by oil pressure due to the quick operation of a throttle, the clutch valve moves in the direction of disconnection in spite of an acceleration state, and the clutch is disconnected. Thereafter, the clutch valve moves in the direction of connection due to the force of the governor caused by an increase in engine speed, and the clutch is connected. That is, when the opening and closing operations of the clutch valve are repeated, it is difficult to transmit power stably.